This invention relates to an electrostatic recording system employed for a facsimile or a printer, and more particularly to a preliminary charging method in a printing system using a single surface control type (Gould type) multi-stylus electrode assembly.
An electrostatic recording method is known in the art in which signal voltages are applied to charging electrodes to apply electrical charges onto a recording medium, which are electrically insulated from one another and arranged in a line, to form an electrostatic latent image on a recording medium such as an electrostatic recording sheet, and the latent image, after being developed with toner power, is fixed. Japanese Patent Application Laid-Open No. 6941/1972 has disclosed a method in which, after an electrostatic recording sheet has been uniformly charged at a certain potential, discharge electrodes of the multi-stylus electrode assembly are grounded according to an image signal, so that the image corresponding to the original picture is erased from the recording medium, and the resultant image, after being developed with toner or the like, is fixed. However, since the method in which the recording sheet is discharged after being uniformly charged is disadvantageous in that the number of drivers is large and accordingly the manufacturing cost is high, and in that it is difficult to uniformly discharge the recording sheet, the above-described method in which signal voltages are applied to the charging electrodes is usually employed at present. A device for applying the desired signal voltages, as disclosed by Japanese Patent Application Publication No. 38531/1976, is commonly employed.
In such a device, auxiliary electrodes are arranged at certain intervals on a surface of a block in which multi-stylus charging electrodes are arranged in an array, and with only the auxiliary electrodes required for printing being driven, electric discharge is caused between the auxiliary electrodes and the charging electrodes by applying a necessary voltage across these electrodes. This generates electric charge so that the electric charge thus generated is held on a recording medium such as an electrostatic recording sheet. Hereinafter, the above-described electrode assembly will be referred to as "a Gould type multi-stylus electrode assembly".
If, in this case, the charging electrodes have positive polarity, then it is necessary to apply a voltage of at least +350 V to the charging electrodes and a voltage of at least -350 V to the auxiliary electrodes; that is, a voltage of at least 700 V must be developed across the charging electrodes and the auxiliary electrodes.
In the method in which positive signal voltages are applied to the charging electrodes, if the dot density of the charging electrodes is increased, electric discharge is liable to occur between the stylus charging electrodes. For instance, in the case of a dot density of 8 dots/mm, electric discharge is liable to occur between the electrodes at a potential of about +350 V or higher. That is a serious problem in the maintenance of the multi-stylus electrode assembly.
In developing with toner or the like a latent image formed on the recording medium in the above-described method, no true electric charge is present in the background region on the recording layer of the recording material, and therefore the toner is liable to adhere to this region, thus making the latter dirty.